1. Field of the Invention
This invention relates generally to the field of amplitude equalizes for data transmission systems. More particularly it relates to an automatic equalization arrangement for use in data modems.
2. Background of the Invention
It is well known that transmission lines used for data communications have amplitude characteristics which vary widely from line to line. It is also known that the characteristics of a particular line are prone to changing as a result of age, weather, etc. As the speed of data transmission increases, it becomes more and more critical that the transmission line be provided with amplitude equalization in order to enhance the probability of minimizing errors in data transmission. This equalization is best carried out by pre-emphasizing the signal transmitted over the transmission medium at the transmitter output in order to correct the attenuation distortion. The reason for this is that, while the same or better degree of equalization can be achieved in the receiver, additional gain provided by equalization in the transmitter has the benefit of not amplifying channel noise. Thus, transmitter equalization is preferable so that signal to noise ratio degradation does not occur at the receiver.
Although receiver equalization can utilize adaptive filtering techniques which can be advantageously used to correct the transmission line characteristics, it is preferable that at least a coarse degree of transmitter equalization be provided for the reason outlined above. Receiver equalization can then be used as a fine tuning mechanism in order to achieve optimal equalization.
In conventional data communications systems such as those based upon data modems, plurality of such transmitter equalizers may be provided each having a distinct equalization characteristic. Generally, the procedure for setting up such a system consists of manually selecting, either on the basis of line measurements or by trial and error, the fixed amplitude equalizer that best fits the attenuation distortion characteristics of the line. This particular equalizer is then strap selected into the transmitter. This process requires operator intervention and can be quite time consuming. Also, it is evident that such a technique is not very useful in adapting to changing line conditions. Such equalization can only practically be handled by an automatic equalization scheme.
In U.S. Pat. No. 4,489,416 to Stuart, a system for providing automated transmitter equalization is provided. In this patent, a central modem individually polls a number of remotely located modems for data. An adaptive equalizer in the central modem is initially trained to minimize the communication link interference from the remote modem and develops a set of equalizer coefficients (based upon the initial training sequence) which is transmitted to the remote modem and stored there for future use. When the remote modem is later polled, the stored equalizer coefficients are used to set the adaptive equalizer of the remote modem.
Unfortunately, the above arrangement of Stuart requires that the actual coefficients be transmitted over the transmission line for use by the remote modem. Since such equalizers for high speed modems may require 48 or more such coefficients which are expressed as complex numbers, this requires the transmission of perhaps 96 or more 8-bit words before equalization can take place. Furthermore, if such coefficients are transmitted utilizing a highly robust secondary channel (typically operating at 75 bits per second) the transmission of the coefficients alone can take well over ten seconds to achieve. In addition, the Stuart patent requires that a special equalizer training sequence be transmitted in order to determine those equalizer coefficients. This further increases the overhead of the protocol resulting in further loss of efficient utilization of the transmission media.
The present invention alleviates these and other problems associated with the prior technology.